THERMAL-STABILITY AND ACTIVITIES OF SOIL ENZYMES AS INFLUENCED BY CROP ROTATIONS

Soil samples were collected to a depth of IO cm in 1991 and 1993 from a vegetable crop rotation experiment initiated in 1989. The two croppi ng treatments, with either 0 or 280 kg N ha(-1), represented the tradi tional vegetable rotation (TVR) and an alternative legume vegetable ro tation (LVR) when a vegetable crop alternated with a red clover (Trifo lium pratense L.) seed-crop that was incorporated as green manure in t he following spring. The enzyme activities of L-asparaginase, amidase and beta-glucosidase were determined on whole soil and five soil aggre gate size fractions: 1.00-2.00, 0.50-1.00, 0.25-0.50, 0.10-0.25 and <0 .1 mm. Thermal stability of the enzymes was determined by conditioning soil samples at 85 degrees C for 2 h or by exposing soil samples to f ive successive freeze-thaw cycles prior to enzyme assays. Enzyme activ ities for LVR were significantly greater (P < 0.05) than TVR for beta- glucosidase and amidase at both N-rates in 1991. This difference in ac tivity for these enzymes was sustained only at the high N-rate in 1993 . The activity of L-asparaginase was significantly higher (P < 0.01) i n the LVR in 1993. Sixty to seventy percent of the soil enzyme activit y (on a mass basis) was associated with macroaggregates with specific distribution of activity across aggregate fractions varying with enzym e. Average beta-glucosidase activity decreased by 50% from 1991 to 199 3. In contrast, amidase activity increased 1.43-fold over the same per iod. beta-glucosidase activity was sensitive to temporal trends, showi ng proportional decreases in activities in each system that were consi stent with decreases in organic C. Furthermore, beta-glucosidase activ ity showed significantly higher (P < 0.05) resistance to heat-induced thermal stress in the LVR in both sampling years. Amidase and L-aspara ginase activities showed no such treatment effects. The results showed that soil enzyme activity is a sensitive biological indicator of the effects of soil management practices.